Allen Jr. is the son of Allen O'Neil, and much like his father he has become a soldier. Seeking revenge on the P.F. Squad for their victories against the Rebel Army, he has gathered Rebel troops and infiltrated the P.F.'s training camp located on a remote island, apparently intending to destroy the whole place.
Allen Jr. has an unknown relationship with the rebel officer Julia; however, given the fact that his special variant has a tattoo that states "Love, Julia", and that Julia mimics Allen Jr.'s behavior, it is assumed that the two are on a date. The two have often been paired up together for missions.
Metal Slug Advance
Allen Jr. made his first appearance in Metal Slug Advance. Acting as a Drill Instructor, he was in fact a spy for the Rebel Army, as was later revealed during his betrayal of Regular Forces on the island. He tricks the player into falling into a trap that leads to the third mission's underground setting, and the player later fights him in the final mission if the correct path is taken. The card earned for beating him shares his namesake, but, unlike his father, his M60 Machine-gun cannot be taken when he is defeated.
Metal Slug Attack
Allen Jr. made his first appearance in the Extra Ops "Boy Memories", where he confronted Walter Ryan when he attempted to save several POWs that Allen Jr. captured. Allen Jr. sent the Cabrakan to kill Walter, but Walter critically damaged it, causing the Cabrakan to destroy the platform that Allen Jr. was on, presumably killing him. However, it is revealed that he survived, as he fell into a Sand Marine. Abigail was then sent to find him, but to no avail. It is presumed that he was captured by the Ptolemaic Army, but he managed to escape and reconnect to Rebel command.
Allen Jr. later appears in "Same Direction" battling Walter, where he promptly retreats and abandons his men after being defeated by him. While taking a shower at base as if nothing happened, Julia storms in, causing him much embarrassment. Julia yells at Allen Jr. for his cowardice, and that it has caused their secret weapons, the Keesi III+ and Formor_ to be captured by the Regular Army. She forces him to rejoin the battle, which he participates in. Upon routing the Regular Army forces, Allen Jr. manages to persuade his soldiers that he retreated abruptly so that he could get Julia's aid. He later gets embarrassed by Julia again in front of all the soldiers.
In "Auto Battle", General Morden asks him to participate in a practice simulation with the Amadeus Syndicate, which he admits that he would have ignored if it came from anyone else. He is accompanied by both Julia and Emma, and battle their opponents - a large unmanned division led by an Amadeus-built Shoe and Karn. During the battle, he states that it isn't the same fighting machines, and soon runs out of ammo. Emma makes him more weapons to continue the fight, but before he can do so, Shoe and Karn end the simulation, as continuing the fight is pointless. Allen Jr. is content with their surrender, as it is still a win in his book.
Allen Jr. made his first appearance in the story "The Pharaoh Treasure", where he's tasked with Beatriz to find the fabled Arabian Tear. They meet Scotia Amundsen during the mission and eventually secure the Arabian Tear.
In the story "The Back You Can't Reach", Allen Jr. is deployed with Beatriz to steal a powerful prototype weapon from the Regular Army. They successfully do so, but eventually fail due to Beatriz unintentionally luring the P.F. soldiers to their base and the destruction of the Cabrakan.
Allen Jr. makes a brief appearance in the story "Cross/You", where he asks Vita to bring him his medicine, and tries to dissuade her from drinking the suspicious liquids when she tries to prove her point but fails.
While Allen Jr. shares some similarities with his father, he is the polar opposite of him - Allen O'Neil's tactics are limited to running into battle with all guns blazing, but Allen Jr. is cool and calculating, as evidenced by his spying on the Regular Army. However, he has a similar fighting style to his father - when he has his M60 he will keep his distance but deliver a blow to the player with his ammunition belt if they get too close. If he takes enough damage, however, he flies into a berserker rage and slashes the player with a Bowie Knife.
In Another Story, Allen O'Neil describes his son as someone that refuses to listen to him, especially during the heat of battle.
His design is based on both Antonio Banderas and Rambo. He uses a M60 Machine-gun has Lewis MG's buttstock, while his father uses an M240 Bravo Machine-gun.